The invention is generally in the field of well logging and relates particularly to a machine-implemented process and a system for using dipmeter logs, other logs, and user interaction to detect and map the structural and stratigraphic make-up of relevant parts of subsurface formations which are most likely to be particularly important in the search for and exploitation of underground resources such as hydrocarbons. Examples are subsurface faults and unconformities which are likely to be hydrocarbon traps, and the structural and depositional environment and stratigraphic characteristics thereof, such as, in a particular example, the structural dips immediately above and below a fault, the nature of the fault (e.g., growth fault), the orientation of the fault (e.g., oriented along a line from 60.degree. to 240.degree., with the downthrown block at 150.degree.), and the stratigraphy (e.g., a distributary-front with an associated channel having channel access at 160.degree. in which the flow was at 75.degree.).
Characteristics of prehistoric beaches, deltas, faults and other subsurface formations can serve as important guides in the search for and exploitation of underground resources such as hydrocarbons, but of course cannot be observed directly in the typical case. Accordingly, their presence and characteristics must typically be found from various indirect measurements, such as those produced by logging a borehole traversing the formations. One of the important logging tools is the dipmeter, which produces dipmeter logs allowing for mapping the dip (i.e., the three-dimensional orientation or attitude) of subsurface formations. Other logging tools produce logs of subsurface characteristics such as electrical resistivity and conductivity, spontaneous electrical potential, nuclear activity and characteristics, and yet other logs are derived in a more indirect manner and comprise measurements versus depth in the borehole of subsurface parameters such as porosity. Such logs can be recorded on a record medium such as film or paper as traces showing the magnitude of the logged parameter versus depth in the borehole. Some, such as the dipmeter logs, can be converted to other presentations, such as an arrowplot or tadpole log comprising a vertical arrangement of arrows, where the vertical position of an arrow corresponds to depth in the borehole, the horizontal position is indicative of dip magnitude and the direction of the arrow is indicative of dip azimuth.
Well logs derived for the same subsurface formations, for example from the same borehole, can be examined by expert log interpreters to seek clues to subsurface formations which are likely to serve as traps for hydrocarbons and to the nature of such formations. A difficulty with such subjective interpretation is that few experts can consistently arrive at accurate conclusions, and that such process is excessively time consuming and prone to error. Accordingly, work has been done in the past toward seeking to find an accurate and consistent way of utilizing well logs to find and map the subsurface characteristics considered important in the search for and exploitation of hydrocarbons, and an example is disclosed in U.S. patent application Ser. No. 140,578, now U.S. Pat. No. 4,414,656, filed on Apr. 15, 1980 in the name of Vincent R. Hepp and entitled "Well Logging System for Mapping Structural and Sedimentary Dips of Underground Earth Formations," which is assigned to the assignee of this application and is hereby incorporated in this application by reference as though fully set forth herein. In addition, proposals have been made in fields other than well logging for utilizing some initial measurements or information respecting a system to derive more general conclusions about its nature. For example, Duda, R. et al, "Development of a Computer-Based Consultant for Mineral Exploration," SRI Report, October 1977, propose mapping subsurface formation from seismic measurements; Buchanan, B. G., Feigenbaum, E. A., "Dendral and Melta-Dendral," Art. Intel., 11:5-24, 1978, propose reaching conclusion about mass spectrum from initial measurements; Reddy R. et al, "The HEARSAY Speech Understanding System," Proc. 3rd IJCAI, 185-193 propose chess moves solutions; Lesser, V. R. et al, "Organization of the HEARSAY-II Speech Understanding System," IEEE Trans. ZXXP, 23:11-23, January 1975, propose techniques in speech analysis; and Shortlifle, E. A., "Computer-Based Medical Consultations: Mycin," 1976, proposes a system for assisting medical diagnosis. See also, Davis R., Buchanan, B. G., Shortlifle, E. H., "Production Rules as a Representation," Art. Intel., 8:15-45, 1977. Unlike such prior art systems in fields other than well logging, the well logging environment has both weak a priori constraints on the ultimate results and weak consistency between result components. For example, multiple geological forces can be at work simultaneously or can affect a given formation in different or same ways at different times, and thus well logs tend to reflect the unseparated influence of such multiple forces; the logging process must contend with highly adverse measurement conditions (in a deep borehole typically having uneven walls and filled with high pressure and often high temperature mud); and the presence and nature of a geological feature at a given depth suggests little about the nature of formations spaced therefrom. Accordingly, in well logging subjective interpretation rather than automated systems have been the known way of utilizing primary and processed logs for finding the subsurface features of greatest interest, such as hydrocarbon traps, as discussed for example in "Fundamentals of Dipmeter Interpretation," 1970, and "Dipmeter Interpretation,", Vol. 1, Fundamentals, 1981, published by the assignee of this application and collectively referred to hereafter as the "dipmeter fundamentals books".
In contrast to having to rely solely on human interpretation of well logs, but while utilizing empirical knowledge developed through such log interpretation, the invention makes it possible for a machine to use the logs to reach the ultimate results considered important in searching for and exploiting subsurface hydrocarbon traps, and to utilize such results to generate subsurface maps of the features of greatest interest in such search and exploitation. In an exemplary embodiment, the invented system comprises a log storage which stores the dipmeter and other logs used in practicing the invention, a blackboard storage which stores the current state of interpretation developed as a result of the invented process as well as the history thereof, an inference engine which responds to the logs and/or patterns in the blackboard storage by changing the state of interpretation therein, and an user-interface terminal. The terminal has a changing display of the most relevant logs and state of interpretation parameters, and inputs by which a user can actively participate in the developing interpretation and mapping of the relevant subsurface formations.
In a particular example the log storage stores the dipmeter and borehole deviation logs and other logs such as the caliper and porosity (e.g., sonic and density) logs and possibly lithology and resistivity logs. The display at the user terminal shows: a small section, e.g. 200 feet, of a selected log, such as the caliper log; the dipmeter log, in the form of an arrowplot, for the same 200 foot section of the borehole; the borehole deviation log for the same 200 foot borehole section; a compressed dipmeter log, in the form of an abbreviated arrowplot showing depth and dip magnitude but not azimuth for the entire borehole interval which has been logged or is of interest, with an indication thereof for the 200 foot section displayed in detail; and other parameters such as a list of types of subsurface formations (e.g., patterns, lithologies, faults, zones, etc.), attributes (e.g., top and bottom borehole depth levels of a particular formation and its nature, or the attributes of a fault such as strike), and a menu of possible user selections. As an initial step, the system makes a first pass through the logs in the log storage to find any depth zones of doubtful quality due to, for example, malfunction of the log tool, logging operator errors, or log portions missing for other reasons. Unless a user selects a depth zone of special interest, the system then treats the logs to find the likely structural dip of subsurface formations. A premise is that small scale processes (river flow, tidal action at shorelines, etc.) produce characteristically varying patterns which typically extend over, say, 10 to 100 feet, while large scale processes (uplift or subsidence of a whole region) contribute additional, relatively constant dip, that extends, say, 500 to 1,000 or more feet. This additional and relatively constant dip is referred to as structural dip and typically results from tectonic as opposed to depositional or erosional events. Structural dip is important for at least two reasons in this invention: it is the overall orientation of large collections of subsurface layers and this in turn is suggestive of the likely direction of flow of hydrocarbons, which tend to float up through porous materials; and the structural dip is a background signal which can be removed from the overall dip of sedimentary layers to assist in the correct interpretation of small scale or stratigraphic events. Discontinuities of certain types in structural dip are indicative of missing sections, which can be faults or unconformities. A fault can be generally defined as a subsurface fracture with subsequent movement of the two blocks relative to each other, and is characterized by a number of attributes, such as the relative movement of both blocks with regard to the vertical and with respect to the dip of the fault plane, by the magnitude of dip of the fault plane, by the time of occurrence with respect to deposition, by the process which caused the fault, by relation to adjacent bedding, etc. An unconformity can be generally defined as the result of a hiatus or significant change in the normal geological sequence caused, for example, by a break in the process of deposition or erosion, or by structural deformation. It generally results in a missing amount of sediments corresponding to a missing geological time as compared to the normal sequence and is generally made up of two different series of strata separated by a surface of unconformity. An unconformity can similarly be characterized by a number of attributes, such as by type (e.g., nonconformity, paraunconformity, disconformity and angular unconformity), lateral extent, missing geological time, etc.
Particularly in the missing sections which could be of interest because they could serve as potential traps for hydrocarbons, it is important to know the depositional environment and the stratigraphy of the subsurface formations and, accordingly, the system utilizes the current state of interpretation (structural dips, faults and unconformities and their attributes) and logs such as, for example, resistivity and lithology, to find the relevant depositional environment (e.g., whether the particular subsurface formation of interest was formed in a marine environment and if so, how deep the water was) and stratigraphy (e.g., the shape of the layer and its orientation and flow direction).
Throughout the process the most relevant aspects of the current state of interpretation contained in the blackboard storage are displayed to the user, and the user can accept, modify or reject intermediate or final interpretation results in the blackboard storage or supply that storage with additional characteristics of the subsurface formations which may be the result of, for example, general and subjective knowledge about the formations of interest. For example, through the interface terminal which displays the intermediate results of finding zones of doubtful validity, the user can accept, modify or reject the relevant automatically derived intermediate results so that the current state of interpretation stored in the blackboard will thereafter incorporate any modifications so made by the user and such modifications can become the basis for later-derived intermediate or final results. Similarly, the structural dip automatically found by the system can be accepted, modified or rejected by the user through the interface terminal, with similar consequences, and the same applies to any other intermediate or even final result automatically derived by the system. As an end result, the system can produce tangible representations of the final state of interpretation contained in the blackboard storage, for example in the form of a map and/or other showing of the relevant part or parts of the subsurface formations, together with any desired intermediate results of the process and characteristics of the nature of the subsurface feature(s) of interest, the likely geological history thereof and possible other relevant attributes thereof.